qom: Correct object_property_get_int() description

The description of object_property_get_int() stated that on an error
it returns NULL. This is not the case and the function will return -1
if an error occurs. Update the commented documentation accordingly.

Reported-By: Christian Liebhardt <christian.liebhardt@keysight.com>
Signed-off-by: Christian Liebhardt <christian.liebhardt@keysight.com>
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

include/qom/object.h

@@ -1115,7 +1115,7 @@ void object_property_set_int(Object *obj, int64_t value,
  * @name: the name of the property
  * @errp: returns an error if this function fails
  *
- * Returns: the value of the property, converted to an integer, or NULL if
+ * Returns: the value of the property, converted to an integer, or negative if
  * an error occurs (including when the property value is not an integer).
  */
 int64_t object_property_get_int(Object *obj, const char *name,